256 CHARLES W. METZ 



and of Ishikawa on Dahlia strongly indicate that the former is 

 the case in some instances. The first author describes a definite 

 and intimate paired association as a normal condition in vege- 

 tative prophases of Spinacia (Stomps '11, p. 258).^^ On the 

 other hand Miiller ('12), who gives perhaps the most complete 

 description of prophase stages in any plant exhibiting paired 

 chromosomes (Najas marina), apparently considers the chromo- 

 somes to be single (though split) in prophase, and believes the 

 real pairing to occur in metaphase. I am inclined to be skeptical 

 about this interpretation, however, for the dual elements in 

 his prophase figures bear a striking resemblance to those in the 

 Diptera. Unfortunately his prophase figures do not include 

 all of the chromosomes in a nucleus, and it is impossible to tell 

 whether the number of double threads is haploid or diploid. 



With respect to the other cases (among plants) in which 

 pairing has been described, the evidence regarding prophase 

 processes is still less satisfactory, and no conclusion of any 

 weight can be drawn from it. The meagre data available from 

 botanical sources tell little about the details of pairing, but they do 

 indicate that it varies in extent or degree among different groups 

 of plants. 



In respect to animals a similar generalization may be made, 

 for, although a conspicuous and uniform pairing seems to occur 

 only in the Diptera, yet a varying degree of pairing is discoverable 

 in other organisms. For instance in Hemiptera the 'm-chromo- 

 somes' and other morphologically distinct types are frequently 

 associated in pairs, and in the Orthoptera a tendency toward 

 pairing has been noted by Montgomery, Sutton and others. 



1" "Spinacia oleracea hat in den vegetativen Kernen ihrer diploiden Generation 

 12 Chromosomen aufzuweisen. Diese sind in Paaren angeordnet, und zwar 

 nicht nur innerhalb der Kernplatten (fig. A.) sondern auch, wenn die Chromo- 

 somen in den Prophasen an der Kernwand liegen und sehr wahrscheinlich auch 

 im Ruhezustande der Kerne. Denn sobald die Chromosomen sich in der Pro- 

 phase einer Teilung aus dem Netzwerk des ruhenden Kerns herausgesondert 

 haben (Prochromosomen sieht man im Ruhekern nicht) zeigen sie die paarweise 

 Anordnung und bisweilen kann man beobachten, wenn in irgendeinem Paare 

 ein Teil der beiden Chromosomen noch mehr oder wenier netzformig ist, dass 

 diese beiden netzformigen Partien einander deutlich parallel liegen." 



